Variable frequency power converter for ac motor drive

Abstract

In a variable frequency power converter of a current type for driving an AC motor including a rectifier, an inverter, and a smoothing reactor disposed on a DC transmission line between the rectifier and the inverter, there are provided a series circuit having a switching circuit and a smoothing capacitor on the DC input side of the inverter and a feedback circuit having controlled rectifier elements and commutation reactors for feeding back the reactive power of the AC motor. The power converter operates as a current type converter until the output frequency of the converter reaches a predetermined value under the condition that the switching circuit and the controlled rectifier elements are non-conductive, and operates as a voltage type converter when the output frequency has reached the predetermined value under the condition that the switching circuit and the controlled rectifier elements are conductive.

I claim: 
     
       1. A power converter for driving an AC electric motor comprising a converter connected to an AC power source for converting an AC power into a desired DC power, a smoothing reactor for smoothing the DC output of the converter, an inverter for receiving the DC output smoothed by the smoothing reactor and for producing a rectangular output current of a desired frequency, an AC motor driven by the inverter, a series circuit connected to the inverter on its DC input side and consisting of a switching circuit and a smoothing capacitor which operates, when the output frequency of the inverter has reached a predetermined value, to compose a smoothing circuit with the smoothing reactor, and a feedback circuit consisting of controlled rectifier elements and commutation reactors which operates, when the output frequency of the inverter has reached the predetermined value, to feedback the reactive power of the AC motor. 
     
     
       2. A power converter for driving an AC motor according to claim 1, in which the switching circuit comprises switching means including controlled rectifying elements and an impedance element connected in parallel with the switching means and the smoothing capacitor is adapted to be charged through the impedance element when the switching means is not operating. 
     
     
       3. A power converter for driving an AC motor according to claim 2, in which the impedance of the impedance element is selected such that the charging voltage of the smoothing capacitor becomes approximately equal to the average value of the DC input voltage of the inverter at the operating time of the switching means. 
     
     
       4. A power converter for driving an AC motor according to claim 1, in which one terminal of each of the commutation reactors of the feedback circuit is connected to the controlled rectifying element side and the other terminal thereof is connected to the AC input side of the AC motor. 
     
     
       5. A power converter for driving an AC motor according to claim 1, in which one terminal of each of the commutation reactors of the feedback circuit is connected to the AC output side of the inverter and the other terminal thereof is connected to the AC input side of the AC motor. 
     
     
       6. A power converter for driving an AC motor according to claim 1, in which a number of AC motors are connected in parallel. 
     
     
       7. A power converter for driving an AC electric motor comprising a converter connected to an AC power source for converting an AC power into a desired DC power, a smoothing reactor for smoothing the DC output of the converter, an inverter means for receiving the DC output smoothed by the smoothing reactor and for producing the rectangular output current of a desired frequency at least during start up of an AC motor, an AC motor driven by the inverter means, and control means for controlling the inverter means when the output frequency of the inverter means has reached a predetermined value so that the inverter means produces a rectangular output voltage, the control means including a series circuit connected to the inverter means on its DC input side and consisting of a switching circuit and a smoothing capacitor for forming a smoothing circuit when the output frequency of the inverter means reaches the predetermined value, and a feedback circuit consisting of controlled rectifier elements and commutation reactors for feeding back the reactive power of the AC motor when the output frequency has reached the predetermined value.